Municipal wastewater, or domestic sewage, is composed of water borne waste derived from household uses and light industry. This includes human waste and wash waters as well as oils, greases, and animal and vegetable matter. Heavier industrial sources, such as breweries and pulp mills, typically have their own treatment or pretreatment plants because their waste generally would overload the usual municipal facility. Domestic sewage can contain 1000 to 2000 parts per million of solid materials with about half of the material being in solution and half in suspension or floating. The contents of industrial waste vary for different processes of manufacture, and the percentage of solid material in sewage from an industrial plant may be very small or inordinantly large. Where storm rain water is to be led away through sewers, openings in the street gutters afford entrance to the sewers. The same sewer may carry all three types of sewage, i.e., domestic, industrial and storm, in a combined system.
Raw sewage from the sewer mains may contain sticks, leaves, rags, paper, glass, sand, grit, cinders and other objects, all of which are liable to clog or injure pumps. In order to ensure satisfactory operation of the pumps, such materials must be removed before the sewage is admitted to the pumps. Large floating objects are usually removed by means of racks or screens that consist of flat bars of iron or steel set a short distance apart and with the wide faces of the bars parallel. Since the purpose of installing the screen or rack is to remove the larger solids, the bars of the screen are placed 1/2" to 3" apart. As stated above, these bar screens are used primarily to remove from the sewage sticks, rags, or other materials that may clog pumps or may interfere with the operation of devices for treatment. They are often used in conjunction with a grit chamber placed at either end of the chamber, but generally is at the inlet end. The width and length of the screen are such as may be necessary to provide for the expected maximum flow of sewage with the screen usually set with the bars sloping in the direction of the flow of sewage.
Typically, bar screens must be cleaned on a frequent basis. Bar screens that are not equipped with an automatic cleaning device require an attendant to rake off the screen whenever enough material has accumulated on the screen to interfere with the flow. Hand raking of the screens is an inefficient use of time and manpower. Accordingly, automatic raking mechanisms have been developed for use with bar screens which can be controlled by a timing device for executing cleaning cycles at predetermined periods and/or based on predetermined operating conditions and thus do not depend on the presence or effort of an attendant. An automatic raking mechanism in a cog rake bar screen apparatus is disclosed in U.S. Pat. No. 5,246,573 commonly assigned to the assignee herein. Cog rake bar screens have a rake and drive assembly with rakes connected to a torque tube in which a main transmission shaft is rotatably mounted by bearings. The drive shaft has cog wheels on either end thereof for cooperating with cog pins disposed in substantially vertical side frame members mounted over the screen upstream therefrom. The entire rake and drive assembly moves up and down the side frame members as the drive shaft rotates during a cleaning cycle for raking of the bar screen.
While cog rake bar screens provide substantial advantages in terms of manpower efficiency over the manual cleaning methods previously employed, they can also encounter maintenance problems. One particular problem faced with cog rake bar screens is the failure of the bearings utilized with the main drive shaft. For supporting the drive shaft for rotation, conventional metal contacting roller element bearing units are provided on either end of the shaft. One of the primary benefits and features of the cog rake bar screen enabling significant commercial success in recent years is that there are typically no moving parts located below the water surface, and the main drive and rake support assembly including the drive shaft and torque tube with the conventional bearings were always located above the operating floor so that these parts were readily accessible for maintenance purposes. However, many recent applications require that cog rake bar screens be installed in multiple story installations or remote pumping stations, such as near rivers which tend to flood. When utilized in these types of installations, the cog rake bar screen drive carriage has the potential to be submerged during its cleaning cycle such as when the rakes travel towards the bottom of the screen. As the bearings require grease lubrication and thus the use of springloaded automatic lubricators which can be mounted to the drive assembly, they experience significant problems when submerged in wastewater flows. The harsh wastewater environment includes exposure to wide ranging water temperatures from approximately 25.degree. C. to -5.degree. C. and exposure to a variety of different corrosive chemicals that can be found in and added to wastewater. It has been found that when the bearings are submerged, the flow of water or sewage tends to flush the grease from the bearings, leading to rapid surface wear and corrosion and additional maintenance requirements. In addition, no current commercially available lubricators are made for fully submerged service. All these factors lead to repeated failures when the cog rake bar screen drive carriage is utilized in applications where the drive assembly is likely to be submerged in wastewater exposing the bearings and lubricators therefor to the harsh wastewater environment. When the bearings fail, the entire drive carriage must be removed from the frame of the cog rake bar screen and disassembled in order to replace the drive shaft support bearings.
Other places where conventional metal-to-metal contacting roller element bearings are not acceptable for use because of exposure to water requiring frequent maintenance is in flocculation mixing tanks. Chemicals are added to the sewage as coagulants to improve sedimentation with thorough mixing of the coagulant chemical with the sewage being desirable in order to obtain economy in chemical dosage. Gentle mixing is also generally necessary in order to obtain a floc, or coagulant, that is large enough to settle. After the floc has been formed, provision must be made for a quiescent period, during which the floc can settle and carry down with it the suspended and colloidal matter in the sewage. These conflicting requirements are best met by providing a separate mixing tank which is apart from the sedimentation basins. The agitating and stirring devices are installed in this mixing tank, the effluent from which is discharged into the settling tanks. Mixing tanks can include both a flash mixer in which a turbulent motion is produced for a comparatively short time by a high speed impeller with flocculation taking place in flocculation tanks having slowly turning sets of paddle wheels which give the chemically treated sewage a slow rolling motion causing the wellflocculated flocculated sewage to return periodically to the part of the chamber in which the sewage is not so completely flocculated to keep it in the mix. Typically, several such paddle wheels are axially aligned and are driven by a common drive shaft, and a mixing tank can include several sets of axially aligned paddle wheels. As the shaft bearings are submerged and continually exposed to wastewater and the coagulating chemicals added to the tank, conventional rolling element bearings are subject to failure and require frequent maintenance and repair when used in this setting. Rigid sleeve bearings, bronzed bushed or babbitted, have also been utilized in this service; however, they require constant greasing via grease lines extended to the top of the basin walls along with grease grooves in the bearing assembly inside diameter. Accordingly, there is a need for a bearing assembly for use with a water treatment apparatus which is not subject to the high maintenance and failures conventional bearings experience when submerged and exposed to the harsh wastewater environment. An easy, less burdensome method of performing maintenance on such a bearing assembly would likewise be desirable.